To add a new item or edit an existing item in the DomainDataSource that ships with the .NET RIA Services, you must first cast the DataView property to an IEditableCollectionView. Jeff Handley does a good job of explaining the reason for this in his DomainDataSource.DataView post.

David Yack left a comment on the .NET RIA Services forum with a link to his post where he shares his inherited DomainDataSource class. The inherited class adds methods for adding and editing items that do all the casting to IEditableCollectionView for you. I really liked the idea but didn’t want to create my own derived class, so I took his implementation and turned it into a set of extension methods that are exposed on all DomainDataSource instances.

Update (24-09-2009): Added a Remove extension method. Thanks to Phil Steel for posting the code in his comment.

/// <summary>
///    Extensions to the <see cref="DomainDataSource"/> for adding and editing items.
/// </summary>
public static class DomainDataSourceExtensions
{
    /// <summary>
    /// Adds a new item to the collection.
    /// </summary>
    /// <typeparam name="T">The type of the item to add.</typeparam>
    /// <param name="source">The <see cref="DomainDataSource"/> being extended.</param>
    /// <returns>The newly added item.</returns>
    public static T AddNew<T>(this DomainDataSource source)
    {
        IEditableCollectionView collection = ((IEditableCollectionView)source.DataView);
        return (T)collection.AddNew();
    }

    /// <summary>
    /// Edits an item in the collection.
    /// </summary>
    /// <param name="source">The <see cref="DomainDataSource"/> being extended.</param>
    /// <param name="itemToEdit">The item to edit.</param>
    public static void EditItem(this DomainDataSource source, object itemToEdit)
    {
        IEditableCollectionView collection = ((IEditableCollectionView)source.DataView);
        collection.EditItem(itemToEdit);
    }

    /// <summary>
    /// Removes an item from the collection.
    /// </summary>
    /// <param name="source">The <see cref="DomainDataSource"/> being extended.</param>
    /// <param name="itemToRemove">The item to remove.</param>
    public static void Remove(this DomainDataSource source, object itemToRemove)
    {
        IEditableCollectionView collection = ((IEditableCollectionView)source.DataView);
        collection.Remove(itemToRemove);
    }

    /// <summary>
    /// Commits the add or edit transaction.
    /// </summary>
    /// <param name="source">The <see cref="DomainDataSource"/> being extended.</param>
    public static void CommitNewAndEdit(this DomainDataSource source)
    {
        IEditableCollectionView collection = ((IEditableCollectionView)source.DataView);
        if (collection.IsAddingNew)
        {
            collection.CommitNew();
        }
        if (collection.IsEditingItem)
        {
            collection.CommitEdit();
        }
    }

    /// <summary>
    /// Cancels the add or edit transaction.
    /// </summary>
    /// <param name="source">The <see cref="DomainDataSource"/> being extended.</param>
    public static void CancelNewAndEdit(this DomainDataSource source)
    {
        IEditableCollectionView collection = ((IEditableCollectionView)source.DataView);
        if (collection.IsAddingNew)
        {
            collection.CancelNew();
        }
        if (collection.IsEditingItem)
        {
            collection.CancelEdit();
        }
    } 
}

Thanks for sharing the code David.

I have been reading up on the new features in Silverlight 3 over the last couple of days, and have often been confused because what I was reading didn’t line up with what I was seeing. It appears many of the articles and videos that are currently available have been created against the Silverlight 3 Beta version, and a number of changes have been made between the Beta and final release. For example, the DataForm control is often mentioned as being in the System.Windows.Controls.Data.DataForm.dll assembly, but has actually been moved from the Silverlight SDK into the Silverlight Toolkit.

I found this page on MSDN that lists all the breaking changes in Silverlight 3 has clarified many of the issues for me. It details the breaking changes from both the Silverlight 2 and Silverlight 3 Beta versions. I’m sure that as time goes by more articles will be referring to the release version and things will become less confusing. In the meantime, if what you are reading doesn’t seem to add up, I would start by checking out the list of breaking changes.